It is well known that organic, and in particular cellulosic, materials can be pyrolyzed to produce valuable solid residue products, including charcoal, low-volatile carbon and activated carbon, as well as gaseous and liquid fuels. Exemplary of the prior art that describes such processes is Bowen U.S. Pat. No. 4,145,256; a pyrolysis process specifically adapted for the production of highly activated carbon is disclosed in Bowen and Purdy U.S. Pat. No. 4,230,602.
In such processes, typically carried out in a vertical-shaft reactor, air and/or steam will be introduced into the high-temperature zone where the principal reactions take place, with the specifications for the solid residue product dictating the amounts thereof to be employed. If, for example, the product is to be a high-volatile carbon suitable for use as a charcoal briquette feedstock, the air will be maintained at a practicable minimum, and no steam will be used. For a low-volatile carbon suitable for use as a chemical feedstock, the air will normally be controlled to a rate slightly above its practicable minimum, and again steam will not be required. Finally, to produce an activated carbon, steam will be introduced, and air sufficient to support the endothermic oxidation of the "fixed" carbon residue, and to thereby activate it, will be used.
In any such process, the offgas stream will contain organic oil (condensible) vapors and gas (noncondensible) vapors, water vapor (at least from residual feedstock moisture), and products of thermochemical reactions with any air or steam that is introduced; it will also contain entrained solid particulates. For reasons of practicability, economy and energy conservation, it is a good practice to keep the temperature of the offgas stream within certain limits and, in processes for the production of solid products ranging from charcoal to low-volatile carbon, this can be accomplished by careful control of the amount of air introduced and of the moisture content of the feedstock. In the production of activated carbon products, the feedstock moisture content is the primary controlling parameter.
In such prior art processes, the thermal energy required to pyrolyze, or to pyrolyze and activate, the solid residue, as the case may be, is generated by burning a portion of the combustible gases and vapors that are produced in the conversion process, the air injected being furnished at a rate adequate to support such reactions; very high local gas temperatures, on the order of 2800.degree. Fahrenheit, result. Moreover, horizontal-bed temperature nonuniformity is a significant factor, since it is found that, if the local bed temperature at the points of air-injection is not maintained at a value of at least about 1800.degree. Fahrenheit, the local thermal energy generation (exothermic oxidation reactions) will become unstable, resulting in unsatisfactory variations in the volatility of the solid product. Such instabilities can indeed lead to complete loss of process control.
Exposing a portion of the solid residue to elevated temperatures for an excessive period of time will, of course, cause its complete devolatilization. Therefore, to obtain a product with a higher volatiles content it is necessary to minimize residence times in the high-temperature zone of the reactor and to quench the char quickly, and even then there will be wide variations in volatiles content from particle to paticle. Taking into consideration all of these factors (i.e., short residence times, relatively high minimum stable bed temperature, and very high local gas temperatures), together with the fact that the downward flowing solid residue will be far from a state of thermal equilibrium with the upward flowing gases, it becomes evident that the control of such a process for the production of a solid carbonaceous product having a predetermined volatility specification, on a continuous basis, is most difficult to achieve.
In specific terms, using wood chips in a continuous, long-residence-time process for producing carbon products containing up to about 20 to 21 percent of volatiles (18 percent being considered a good high-volatile product), it has been found that the maximum temperature to which the solid residue can be heated is about 800.degree. to 1400.degree. Fahrenheit, the percentage of volatiles being inversely related to temperature and being expressed on the basis of the weight of solid product, exclusive of all water and water vapor. As a further indication of the close control required, it is noted that a relatively small temperature variation will produce a substantial change in the volatiles content. Thus, heating the residue to about 1200.degree. rather than 1000.degree. Fahrenheit will reduce the volatility level of the product from about 12.7 to 5.7 percent.
In addition to the foregoing, it is self-evident that any air introduced into such a process will cause a corresponding increase in the volume of nitrogen present, and a concomitant dilution of the volumetric heating value of the pyrolytic gases produced. Moreover, the injection of air low in the bed generates high-temperature oxygen compounds, which react with the pyrolytic oil vapors and detrimentally alter the desirable chemical and physical properties of the ultimate oil product.
Accordingly, it is a primary object of the present invention to provide a novel process and system for continuously pyrolyzing an organic material to produce a gaseous product, a pyrolytic oil product, and a solid carbonaceous residue product having a predetermined volatiles content.
It is a more specific object of the invention to provide such a process and system by which a residue product having desirable specified levels of volatile fractions can be produced.
It is a further specific object to provide such a process and system for producing a gaseous product of undiluted volumetric heating capacity, and an oil product of physical and chemical properties which are substantially unadulterated by alien oxygen compounds and/or thermal degradation.
Yet another object of the invention is to provide a process and system of the foregoing nature in which the product gas is sufficiently cleansed of condensible organic vapors and solid particulate that it can be continuously reheated and recycled to the reactor without fouling of the system.
A still further object of the invention is to provide a process and system of the foregoing nature in which the pyrolytic gas produced can be recycled and used to dry and heat the organic feed material to its self-decomposition temperature, to control the temperature of solid residue so as to produce therein a predetermined desired volatility level, to recover thermal energy from the solid residue product to cool it and to afford optional thermal efficiency, and to provide an essentially one-dimensional conversion process for facile control of product variables.
A further object of the invention is to provide a novel process and system having such features and advantages, which are also energy-efficient, convenient and relatively simple and inexpensive to carry out and to use.
An additional specific object of the invention is to provide a method for processing organic material which tends, when heated, to agglomerate or form a tacky mass of low permeability, to produce a solid carbonaceous residue product, an oil product and a gaseous product.